The disclosure relates to glass substrates having surfaces that are oleophobic. More particularly, the disclosure relates to glass substrates having surfaces that are free of multiscale structures such as reentrant, overhang, or fractal structures, and are either oleophobic or superoleophobic.
Non-wetting substrates that are formed through surface chemistry and surface texturing are being increasingly employed in applications such as micro-fluidics, MEMS, hand-held devices, touch screen applications, and the like, in which a surface that is repellant to water and/or organic (i.e., oils) fluids is desired. The non-wetting characteristics of a substrate, such as, for example, a glass substrate, is usually classified in terms of the static contact angle of a small liquid droplet placed on the substrate, wherein the dimension of the droplet is smaller than the capillary length of the liquid. If the working liquid is an oil, the substrate is characterized as being oleophilic (i.e., wetting) if the static contact angle between the oil droplet and the substrate is less than 90°; or oleophobic (non-wetting) if the static contact angle between the oil droplet and the substrate is greater than 90°; or superoleophobic if the static contact angle between the oil droplet and the substrate is greater than 150°.
Surface roughness can, in some instances, enhance the wetting and/or non-wetting characteristics of a substrate. If the static contact angle of a liquid on a flat surface is greater than 90° or if the original flat surface is non-wetting with respect to the liquid, that surface could be made even more non-wetting with respect to the liquid by adding roughness to the surface. Certain material surfaces, such as TEFLON™ are non-wetting with respect to water (contact angle ˜110°) even when the surface is perfectly flat. Such substrates can be made superhydrophobic by providing the substrate surface with a rough geometry that may include simple geometric structures, such as cones, cylinders, posts, or the like.
In contrast, synthetic or naturally occurring oleophobic substrates that are perfectly flat are generally not known. The surface tension of oils and other organic liquids is very low, and the contact angle of oil on a perfectly flat surface is less than 90°; i.e., the perfectly flat surface is oleophilic. Accordingly, the oleophobic state and, particularly, the superoleophobic state are metastable on simple surface geometries such as cones, posts, cylinders, and the like. This metastability leads to a transition on simple rough surfaces to the oleophilic or wetting state. The few known cases of oleophobic and superoleophobic substrates are made through creation of multiscale structures with reentrant, overhang and fractal geometries.